For golfers, it is an eternal theme to extend flight distance of a ball and shoot the ball at an aimed direction and angle. Therefore, it is important to use a golf club suited for one's own swing.
Selecting a golf club suited for a golfer is generally referred to as fitting. In order to effectively perform this fitting, it is necessary to take into consideration various factors such as the total weight of a golf club, the weight of a club head, and the length of the golf club. In particular, the success or failure of fitting is greatly influenced by physical properties of a shaft of a golf club.
For example, one of the physical properties of a shaft is flex. This flex represents the hardness (stiffness) of a shaft. As shown in FIG. 58, when a point that is 129 mm away from a tip end 20a of a shaft 20 is defined as a load point W1, when a point that is 824 mm away towards the butt end side of the shaft 20 from the load point W1 is defined as a fulcrum point A, and when a position that is 140 mm away towards the butt end side from the fulcrum point A is defined as a working point B; a forward flex is a displacement magnitude of the tip end 20a when a load Wt of 2.7 kgf is applied on the load point W1. With respect to this displacement magnitude F1 (this value is referred to as forward flex), flex is determined by defining, as an X shaft, one that has a range from certain value f1 to certain value f2.
Generally, regarding flex, a suited hardness is recommended depending on the level of head speed. A shaft that is easily flexed is recommended for a golfer with a relatively low head speed, whereas a shaft that is hard is recommended for a golfer having a relatively high head speed. However, there is no unified standard for flex, and different standards are defined by every manufacturer. Therefore, selecting a suited flex value depends largely on the experience and intuition of the person (fitter) who performs the fitting, and the selection result is not objective and varies from person to person.
Another physical property of a shaft is kick point. As shown in FIG. 59, the kick point is determined in the following manner. When a position that is 12 mm away from the tip end 20a of the shaft 20 is defined as a working point C, when a point 140 mm away from the working point C towards the butt end of the shaft 20 is defined is a fulcrum point D, and when a point 776 mm away from the fulcrum point D towards to the butt end side is defined as a load point W2; a displacement magnitude F2 (this value is referred to as backward flex) of the butt end 20b when a load Wt of 1.3 kgf is applied on the load point W2 is obtained, T is calculated from values of this F2 and the above described F1 (forward flex) in accordance with the following expression (1), and it is determined whether a shaft has low kick point or high kick point depending on the magnitude of the value of T.T=F2/(F1+F2)×100  (1)
Similar to the case using the above described flex, selecting a golf club using this kick point also has to depend on the experience and intuition of the person who performs the fitting, and the selection result is not objective and varies from person to person.
Therefore, when performing the fitting, it is proposed to ask a golfer to actually swing a golf club, and perform the fitting from a measurement result of the swing (e.g., cf. Patent Literatures 1 and 2).
Patent Literature 1 discloses an evaluation method of a golf swing, and the method includes: a model creation step of generating a golf club model of a golf club used in a golf swing; a property value calculation step of calculating, in order to reproduce a golf swing using the golf club model, a desired dynamic property value of the golf club model in a golf swing by computing a swing action of the golf club model by providing a predetermined boundary condition to the golf club model; a repeat step of obtaining respective dynamic property values for multiple golf club models by repeatedly conducting the property value calculation step while changing the type of the golf club model that is generated; and a swing evaluation step of extracting a maximum value and a minimum value of the dynamic property values among the multiple dynamic property values obtained in the repeat step, and classifying and evaluating characteristics of a golf swing based on the difference between the maximum value and the minimum value.
Furthermore, Patent Literature 2 discloses a swing evaluation method of a golf club including a golf club head provided with, at a crown part thereof, a hosel part at which a golf club shaft is mounted. In the swing evaluation method, respective speeds during a swing of a golf club are measured at two points that are separated by at least 10 mm and are on an intersection line of the crown part of the golf club head and a plane that, when the golf club head is placed on the horizontal plane, contains an axis line of the golf club shaft and that perpendicularly intersects the horizontal plane, and the swing is evaluated based on the speed of each of the points.